NUCLEAR SPIN CONVERSION AND THE ASSIGNMENT OF VIBRATION-ROTATION FINE STRUCTURE OF MATRIX ISOLATED METHANE.
Loading...
Date
1967
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Ohio State University
Abstract
The Pauli Principle places severe restrictions on the nuclear spin states possible for rotational energy levels in molecules with identical nuclei. In methane a nuclear spin quintet is associated with the $J = 0$ level and the $J = 1$ level is a triplet. An infrared study of both the $\nu_{3}$ and $\nu_{4}$ regions of $CH_{4}$ in an argon matrix at $4.2^{\circ} K$ shows fine structure absorption which is time dependent. These changes, with a half life of 225 minutes, are evidence for the forbidden spontaneous triplet $\rightarrow$ quintet nuclear spin transition. Absorpotion features which decrease with time are assigned to $R(1), Q(1)$ and $P(1)$ transitions and the absorption which increased was called $R(0)$. These assignments are compared with King's recent theoretical study of crystal filed effects on rotation of tetrahedral molecules in cubic $lattices.^{1}$ The barrier to rotation of $CH_{4}$ in argon is estimated to be on the order of 1 keal/mole and Coriolis coupling does not appear to be much effected by the matrix. The mechanism of the triplet $\rightarrow$ quintet transition results from external perturbations which mix spin states. The effect of trace paramagnetic impurities which greatly increases the rate of spin conversion will be discussed.
Description
This work was supported by a grant from the National Science Foundation. $^{1}$ H. King and D. Hornig, J. Chem. Phys., 44, 4520 (1966).
Author Institution: Department of Chemistry, Indiana University
Author Institution: Department of Chemistry, Indiana University